1. FIELD OF THE INVENTION
This invention relates to propulsion systems for spacecraft.
2. DESCRIPTION OF THE RELATED ART
The invention is particularly applicable to the type of propulsion system that produces a relatively low thrust in relation to its weight, making it unsuitable for lift-off of a spacecraft, i.e. the type of propulsion system that is suitable for making adjustments to the position and/or attitude of an orbiting spacecraft, and/or for varying the orbit. For this purpose, advantage can be taken of propulsion systems of relatively high specific impulse (a quantity which gives a measure of the impulse which can be produced from a given mass of propellant) which produce a low absolute value of thrust. Included among such propulsion systems are a kind which use electrical power to increase the specific impulse of the propellant by transferring energy into the propellant. Typical of such thrusters are ion engines, arc-Jets and plasma engines. In ion thrusters, the ions are accelerated out of the thruster by electrostatic attraction between aligned grids at appropriate potentials. In arc-jets the exhaust velocity is increased by heating the propellant by generating an arc through it. In plasma engines, including magneto-plasma thrusters and stationary plasma thrusters, the ionised propellant is accelerated by means of a strong magnetic field.
One example of adjustments which must be made to the position of an orbiting spacecraft concerns geostationary satellites, i.e. satellites in orbit around the equator at a radius of approximately 22,300 miles from the earth (FIG. 1). Because the plane of the orbit S is inclined to the plane of the ecliptic E, i.e. the earth's orbit around the sun (the earth's axis being inclined to the plane of its orbit), the orbit tends to be perturbed by the gravitation attraction of the sun, moon and planets, and it is found necessary to apply periodic east-west and, more frequently, north-south corrections to the spacecraft to maintain it in the necessary fixed solid angle as seen from the earth. For such so-called north-south station-keeping, it is conventional to provide a pair of thrusters, both on the north face, or both on the south face, or one on the north face and one on the south face of the spacecraft. In one arrangement where the thrusters do not act through the centre of gravity of the spacecraft, both thrusters must be fired simultaneously in order to avoid generating a moment about the centre of gravity of the spacecraft which would cause it to spin. In another arrangement where the thrusters do act through the centre of gravity of the spacecraft (FIG. 3), no spin can take place, but the thrusters are operated at spaced points in the orbit of the spacecraft to cancel an unwanted east-west component imparted to the spacecraft. It will be seen from FIG. 1 that a north-south correction must be imparted in the direction of the arrows. The north-south direction is also shown on the spacecraft shown in FIG. 2. The thrusters T generate thrust passing through the centre of gravity CG of the spacecraft. In applying a burst to one of the thrusters, a north component is generated, but a radial component is also generated. Build-up of the latter component can be offset by operating the south pointing thruster T at the opposite point in the orbit of the spacecraft.
In all cases, each thruster incorporates its own power supply for transferring electrical energy into the propellant by using propellant gas or vapour as part of the current carrying circuit.